I have 7 kids, so I feel qualified to make some observations on this topic.
Kid #1 asked "why" questions all the time when she was young. As a teenager, her questions have definitely decreased in frequency. This is primarily because all the questions she had as a young child actually got answered. There was a LOT of low-hanging fruit, and she picked it when she was young. She is still curious; her teachers enjoy her genuine interest in learning. It competes with her love of fan fiction, though.
Kid #4 also has some curiosity, and asks questions, though not as often as Kid#1 ever did. He, too, has fewer questions as he ages.
Kids #2, 3, and 5 never actually went through a "why" phase. They ask "Why can't I have that candy bar?" but they don't ask "Why is the sky blue?" They ask practical questions about what, when, where, and they may be quite interested if there's an interesting demonstration of something, but curiosity isn't a big part of their makeup. I have also noticed other people's kids who aren't that curious. People who say that all young kids are curious are basing that on observations of kids who are. Confirmation bias: they aren't looking for kids who aren't curious.
Kid #6 isn't very curious, but she is extremely social and wants to always hear the sound of her voice and mine, so she asks lots of questions and then doesn't listen to the content of the answers. Kid #7 's vocab consists mostly of a handful of food items and "shoes", so her curiosity can't be gauged yet.
So I'd say it's not the case that most young kids are curious and lose that as they grow older. Rather, most young kids are not that curious, and continue not to be curious as adults. The kids who are uber-curious grow up to be adults who are still curious, but whose questions are less incessant because they find answers as they go.